The present invention relates to improvements in papermaking systems and particularly the apparatus employed therein for transferring deaerated papermaking stock from a collecting zone of an evacuated chamber defined by a stock receiver to the paper web-forming means.
Modern methods of papermaking often take advantage of the benefits of employing deaerated papermaking stock suspension as the furnish to the headbox of the papermaking machine or other point of use. Deaeration may be accomplished by atomizing the stock suspension into an enclosed receiver which is maintained under deaerating vacuum. The stock may also be cleaned in centrifugal solids separating means, such as hydrocyclones or other forms of cleaners, and such separating means may advantageously be combined with the deaeration means whereby the dirt-poor or accepts fraction from the separating means is sprayed directly into the evacuated receiver. Plural stages of cleaning may be employed and the accepts fraction from all or some of the cleaning stages subsequent to the primary stage may similarly be deaerated. A more complete description of method and apparatus for deaerating and for cleaning may be found in U.S. Pats. 1,853,849, 2,571,219, 2,614,656, 2,642,950, 2,685,937, 2,717,536, 2,751,031, 2,876,860, 2,931,503, 3,131,117, 3,206,917, Kaiser U.S. Ser. No. 417,161 filed Dec. 9, 1964, now U.S. Pat. 3,432,036, Kaiser U.S. Ser. No. 526,256 filed Feb. 9. 1966, now abandoned, and Kaiser U.S. Ser. No. 694,494 filed Dec. 29, 1967, now U.S. Pat. 3,538,680.
In papermaking systems employing deaerated papermaking stock, the deaerated stock commonly is delivered from an evacuated stock receiver to the paper web-forming means by single relatively small supply conduit means and fan pump or like impeller means connecting the receiver with the headbox of the web-forming means, as shown, e.g., in the Kaiser et al. U.S. Pat. 3,206,917. The supply conduit means are connected to the bottom portion of the collecting zone of the stock receiver at a central location, and the communicating outlet from this zone to the supply conduit occupies but a very small fraction of the lateral expanse of the bottom portion of the collecting zone. At the outlet end of the supply conduit, the papermaking stock is delivered to a manifold for supplying the stock to the headbox of the web-forming means, the manifold extending laterally to both sides of the supply conduit in order to attempt to attain even distribution of the stock along the expanse of the slice in the headbox from whence issues the stream of effluent ribbon of stock cast, for example, onto the traveling wire of a Fourdrinier unit. The conveyance of stock from the stock receiver to the web-forming means headbox in such type of conduit thus involves undesirable alterations in the conduit flow course in a direction laterally of the web-forming means axis particularly in effecting flow course transition from the conduit to outlet at the back of the web-forming means headbox. Further, the passage of the deaerated stock from the receiver to the headbox in a relatively small artery which demands that there be a relatively high flow velocity that may require relatively high pump pressure in order to maintain the transfer flow of a predetermined rate. Certain system operating problems may attend this mode of supplying deaerated stock to the web-forming means, including floccing or coagulation of stock fibers and like stock particles on internal conduit surfaces such as valve closure members, elbows, etc., as well as cavitation at points where flow pressure drops, such as at locations wherein conduit passage size is diminished with concomitant increase in velocity of flow, especially at the papermaker's valve. Cavitation-promoting conditions are desirably eliminated from the system since cavitation, especially at the papermaker's valve, can introduce a marked unevenness of flow to the web-forming means and adverse effect upon the quality and uniformity of paper formed on the wire or other forming means. Also, the magnitude of the pressure drop often causes vibration and shaking of the papermaker's valve and associated parts of the system, which vibration also imparts non-uniformity to the product as well as causing wear and damage to the system structure.
Certain papermaking system improvements have been proposed to minimize the above-described drawbacks in prior art papermaking systems. For example, Kaiser U.S. patent application Ser. No. 526,256, filed Feb. 9, 1966, now abandoned, discloses an improved papermaking system wherein the transfer of papermaking stock from the stock receiver to the headbox of the paper web-forming means is effected by gravity, thus eliminating the need for the conventional stock transfer pump with consequent effective reduction of cavitation as a system shortcoming as well as improvement in flow conditions to and at the paper web-forming means. The desirable end of reducing cavitation as well as improving the consistency and quality of furnish supplied to the web-forming means with the provision of a greater degree of constancy of pressure within the supply conduit is achieved by elevating the stock receiver a height above the delivery slot or slice of the web-forming means that provides a hydrostatic head between the elevation of the surface of the deaerated papermaking stock in the receiver and the elevation of the web-forming level of the paper forming means in excess of that necessary to offset the condition or degree of vacuum within the stock receiver deaerating chamber plus all of the intervening hydraulic losses associated with the supply conduit, papermaker's valve, etc., the overall vertical distance between these two elevations being measured in terms of a dropleg of certain height.